The present invention relates to very thin layers of electrical insulation that may be used to coat and protect microminiature components and devices that are intended to be implanted in living tissue and/or to maintain electrical leakage of such components/devices within acceptable limits, e.g., less than 1 xcexcA/cm2 when the components and/or devices are submerged in water or salt water. More particularly, select embodiments of the invention relate to the use of alumina or aluminum oxide as a safe, biocompatible, coating material that provides a reliable, protective and insulative layer or coating for components, or devices comprised of components, wherein the insulating layers can be made extremely thin, on the order of microns, yet wherein the electrical leakage through the thin insulative layer (when the coated component or device is implanted or otherwise immersed in a saline solution or in distilled water) is less than about 1 xcexcA/cm2 (or less than about 12.1 nA for an area of 0.075 inchesxc3x970.025 inches, corresponding to an area of 0.1905 cm by 0.0635 cm).
The use of alumina as a thick insulator for use with implantable devices has previously been disclosed, for example, in U.S. Pat. Nos. 4,940,858 and 4,678,868 assigned to Medtronic, Inc. In these applications, however, the alumina insulator is very thick and is used only as part of the feedthrough for the implantable device and is often carried by a metal ferrule. Such use of alumina (or other ceramic) as an insulator requires a relatively thick layer. Many materials work well as an insulator when put down in a thick layer, e.g., in a layer thicker than 25 microns (where 1 micron=1xc3x9710xe2x88x926 meter). But all such materials, except as discussed herein, typically leak at a rate greater than about 1 xcexcA/cm2. Applicants invention, as set forth below, uses a nonconductive ceramic, such as alumina, in very thin layers, e.g., less than about 25 microns.
It is also known to use the ceramic alumina as a case material for an implanted device as disclosed in U.S. Pat. No. 4,991,582, incorporated herein by reference. Again, however, the alumina, while comprising a material that is biocompatible (and is thus not harmful to, and is not harmed by, living tissue and fluids wherein it is implanted), is relatively thick, e.g., greater than 25 microns.
A problem with the related art is that the thickness of the insulation needed for implantable devices is typically. on the order of about several millimeters thick. None of the related art, to applicant""s knowledge, has heretofore achieved an insulating layer with very small dimensions and free of micro-holes. The presence of a micro-hole, or xe2x80x9cpin-holexe2x80x9d, destroys the insulating properties which may lead to eventual failure of the implantable device.
Further, some components or devices which need to be implanted in living tissue, such as magnets, are susceptible to extremely high temperatures, i.e., extremely high temperatures may damage or destroy such components. When such components or devices must be implanted, it is important therefore that whatever coating or encapsulating material is used to coat them be one that can be applied without subjecting the component or device to extremely high temperatures. That is, the coating or application process must not subject such components to extremely high temperatures.
It is seen, therefore, that what is needed is a way to utilize a very thin layer of a suitable insulating material, such as alumina (aluminum oxide), zirconia (zirconium oxide), or alloys of alumina and/or zirconia, at relatively low temperatures, as a coating to cover, insulate and/or encapsulate any type of component or device that must be implanted, thereby effectively rendering such coated component or device biocompatible and safe for implantation. In particular, it is seen that what is needed is a very thin insulative coating that can be applied at relatively low temperatures for the purpose of insulating electrical connections on implantable devices and other microminiature devices, or for coating non-biocompatible components (thereby making the coated component biocompatible) wherein the coating can be as thin as about {fraction (1/1000)} of an inch or less yet still maintain the electrical leakage through the insulator at or below acceptable levels.
The present invention addresses the above and other needs.
An aspect of the present invention provides a protective, biocompatible coating or encapsulation material that may be applied to a component or device intended to be implanted in living tissue. The coating or encapsulation material comprises a thin layer or layers of alumina, zirconia, and/or alloys of alumina and/or zirconia. Advantageously, a thin alumina or zirconia layer applied in accordance with the present invention may be applied at realtively low temperature. Once applied, the coating provides excellent hermetically, and prevents electrical leakage, while retaining a microminiature size. The layer of alumina or zirconia insulation can be made as thin as about {fraction (1/1000)} of an inch (Note: {fraction (1/1000)} inch=0.001 inch=1 mil=25.4 microns) or less while still retaining excellent insulating characteristics. For example, in accordance with one aspect of the present invention, an alumina coating having a thickness that is less than about 5-10 microns provides an insulative coating that exhibits less than about 12 nA of leakage current over an area 75 mils by 25 mils while soaking in a saline solution at temperatures up to 80xc2x0 C. over a three month period.
Advantageously, the invention may be used to encapsulate or coat (and thereby insulate) passive electrical and/or magnetic components, such as resistors, capacitors, inductors, wire, conductive strips, magnets, diodes, etc., and/or active electrical components, such as transistors, integrated circuits, etc., and/or assemblies or combinations of such passive and/or active components. Because the coating layer can be made extremely thin, yet still provide the needed insulative properties required for an implanted component or device, the overall size of such components or devices does not increase significantly from the normal size (non-implanted size) of such components or devices. For many applications, e.g., as taught in U.S. Pat. No. 5,193,539, incorporated herein by reference, a complete implanted device, comprised of many different components, may be coated and maintained at a microminiature size. For other applications, e.g., the implantation of one or more permanent magnets, such magnets may be coated with the alumina or zirconia coating, thereby effectively hermetically sealing the magnets in an alumina or zirconium encapsulation that renders the magnets suitable for direct implantation in living body tissue at a desired location.
It is an object of the invention to provide a biocompatible, thin, insulative coating that is easy to apply to a wide variety of different shapes and sizes of components and devices, and that once applied provides excellent insulative properties for the covered component or device over a long period of time, thereby allowing the covered component or device to be safely implanted in living tissue for long periods of time.
It is a further object of the invention, in accordance with one aspect thereof, to provide a biocompatible, insulative coating that may be applied to implantable components or devices of various shapes and sizes, and wherein the coating is: (1) less than about 10 microns thick; (2) submersible for long periods of time in water or saline solution or any other conductive fluids; (3) made from alumina, zirconia or alloys of alumina and/or zirconia, or other substances having properties the same as or similar to alumina, zirconia and/or alloys of alumina and/or zirconia; (4) amenable to being applied using a batch process, e.g., a process wherein 1000 or more devices or components may be coated at the same time using the same process, such as an evaporative coating, vapor deposition, or ion-beam deposition (IBD) process; and/or (5) extremely strong in the lateral direction.